docs: def Chen_1979(Re, eD):
      r'''Calculates Darcy friction factor using the method in Chen (1979) [2]_
      as shown in [1]_.
  
      .. math::
          \frac{1}{\sqrt{f_f}} = -4\log\left[\frac{\epsilon}{3.7065D}
          -\frac{5.0452}{Re}\log A_4\right]
  
      .. math::
          A_4 = \frac{(\epsilon/D)^{1.1098}}{2.8257}
          + \left(\frac{7.149}{Re}\right)^{0.8981}
  
      Parameters
      ----------
      Re : float
          Reynolds number, [-]
      eD : float
          Relative roughness, [-]
  
      Returns
      -------
      fd : float
          Darcy friction factor [-]
  
      Notes
      -----
      Range is 4E3 <= Re <= 4E8;  1E-7 <= eD <= 5E-2.
  
      Examples
      --------
      >>> Chen_1979(1E5, 1E-4)
      0.018552817507472126
  
      References
      ----------
      .. [1] Winning, H. and T. Coole. "Explicit Friction Factor Accuracy and
         Computational Efficiency for Turbulent Flow in Pipes." Flow, Turbulence
         and Combustion 90, no. 1 (January 1, 2013): 1-27.
         doi:10.1007/s10494-012-9419-7
      .. [2] 	Chen, Ning Hsing. "An Explicit Equation for Friction Factor in
         Pipe." Industrial & Engineering Chemistry Fundamentals 18, no. 3
         (August 1, 1979): 296-97. doi:10.1021/i160071a019.
      '''
      A4 = eD**1.1098/2.8257 + (7.149/Re)**0.8981
      ff = (-4*log10(eD/3.7065 - 5.0452/Re*log10(A4)))**-2
      return 4*ff